feat: add drop constraint if exists to compiler

doc/build/changelog/unreleased_20/8141.rst

@@ -0,0 +1,11 @@
+.. change::
+    :tags: usecase, schema
+    :tickets: 8141
+
+    Added parameter :paramref:`_ddl.DropConstraint.if_exists` to the
+    :class:`_ddl.DropConstraint` construct which result in "IF EXISTS" DDL
+    being added to the DROP statement.
+    This phrase is not accepted by all databases and the operation will fail
+    on a database that does not support it as there is no similarly compatible
+    fallback within the scope of a single DDL statement.
+    Pull request courtesy Mike Fiedler.

lib/sqlalchemy/sql/compiler.py

@@ -5210,10 +5210,11 @@ def visit_drop_constraint(self, drop, **kw):
                 "Can't emit DROP CONSTRAINT for constraint %r; "
                 "it has no name" % drop.element
             )
-        return "ALTER TABLE %s DROP CONSTRAINT %s%s" % (
+        return "ALTER TABLE %s DROP CONSTRAINT %s%s%s" % (
             self.preparer.format_table(drop.element.table),
+            "IF EXISTS " if drop.if_exists else "",
             formatted_name,
-            drop.cascade and " CASCADE" or "",
+            " CASCADE" if drop.cascade else "",
         )
 
     def get_column_specification(self, column, **kwargs):

test/sql/test_constraints.py

@@ -1261,6 +1261,16 @@ def test_render_drop_constraint_cascade(self):
             "ALTER TABLE tbl DROP CONSTRAINT my_test_constraint CASCADE",
         )
 
+    def test_render_drop_constraint_if_exists(self):
+        t, t2 = self._constraint_create_fixture()
+
+        constraint = CheckConstraint("a = 1", name="a1", table=t)
+
+        self.assert_compile(
+            schema.DropConstraint(constraint, if_exists=True),
+            "ALTER TABLE tbl DROP CONSTRAINT IF EXISTS a1",
+        )
+
     def test_render_add_fk_constraint_stringcol(self):
         t, t2 = self._constraint_create_fixture()